Really? If I've done the calculation right, the sides of just one U.S. aircraft carrier have an area around 30,000 m^2. There are 12 of them. The sides of the SE in the NIAC proposal, in the section below 100 km [*], have an area around 20,000 m^2. It would be nice to have several of them. It seems to me that each SE has a smaller "profile" than each aircraft carrier.

[*](My understanding is that NASA classifies anyone above 100 km as an astronaut. Do you expect astronauts to attack the SE?)

Aircraft carriers are military ships protected at all times by other military ships, aircraft and missile defense systems.

That's my point exactly. The people on the aircraft carrier seem to think these other ships do an adequate job defending them. It seems to me that a similar group of ships would do a similarly adequate job of protecting the SE.

Aircraft carriers are built to withstand attack.

True. The SE is merely built to withstand a constant barrage of meteoroids.

Aircraft carriers move around and don't publicize their whereabouts.

If Aircraft carriers are so invisible, why does anyone bother making submarines?

Yes, exactly. Even something like an aircraft carrier which can move around and not publicize its whereabouts is too vulnerable, hence the need for submarines. The space elevator is even more vulnerable than the aircraft carrier.

Also assume that it can be defended long enough to make a second one, and both of those can be defended long enough to make a 3rd, and so on. The benefits of cheap orbital lift will then make it very hard for any terrorist organization to ... drop them all.

Is it or is it not the case that there is enough bound energy in the cable that the recoil were it ever cut would cause immense destruction on the ground? I'm pretty sure that it is... Which means the risk is simply too great, for now. If it were just about the risk of losing the investment, that would be one thing.

There is, but you would have to cut it near the top to get it. If you cut it near the ground, not much happens.

So as more are built and orbital lift/travel gets cheaper, the ability of a terrorist or terrorist organization to cut it near the top increases

One of the most important protections against the "Hot Target" status of
the SE, and against economic losses from its destructions by man or Nature,
is to BUILD MORE THAN ONE. After all one of the biggest components of the
cost of the first SE is [rocket] launch costs to put reels, satellites, etc., in
orbit at the top. So once one is in place, making more would be fairly
simple and cheap. ...
it would only be prudent to have multiple SEs available, against the day
(and it may be centuries in the future) when one gets whacked in any
significant way. If there was only one TV transmission tower in the world,
it would be an obvious target. But there are so many, does it even make the
local news when one gets put out of action?
-- Brad Walsh

If a TV tower wrapped itself around the equator, I predict it would make the local news. Lots of people say "such and such laid end to end would wrap around the equator twice", but none of them have the gumption to actually do it.

terrorist psychology?

The benefits of cheap orbital lift will then make it very hard for any terrorist organization to stay popular long enough to drop them all.

Terrorists don't need popularity. Even if they did, the people who profit from cheap orbital lift aren't big fans of terror to begin with.

They need a power base - people who will let then train in their territory, people to fund them,and people to fill the ranks. None of the radical Islamic groups would have any real effect on the world were it not for the large number of young men who agree with their cause and are willing to trains, fight, and die for it. One might draw a parallel with the Luddites - how many people today follow their cause all the way? Few do, because few see any gain from the removal of all mechanized labor from the world.

This is a popular myth. The Unabomber was a terrorist without any popularity. All it takes is one committed and resourceful individual.

The unabomber, while somewhat effective, is not even in the same league as the groups (Islamic and other) mentioned in the above paragraph.

All this is entirely besides the point. The question to ask is, what sort of targets do terrorists attack? Space-related targets don't seem to be high on the list, and for that matter the list is fairly short - an accident would be far more likely than a terrorist attack. And, one might imagine, the answer would depend on what country built the elevator. If the US is too paranoid to make such an investment, Europe or China could do it.

A 62,000 mile elevator will be vulnerable to motivated minor leaguers.

Most of that 62,000 miles are too high in the sky for terrorist, the Earth bound part is just as risky as any skyscraper, the risk of other ground infrastructures is the same as any airport. The best they can do is cut the rope at the ground, which will send it upward until the skyward end compensates for it, then the rope will just dangle in the sky. Then they can just reattach the rope later, what's the fuss? Hitting a skyscraper will probably give more publicity, unless they managed to do it at the open ceremony of the space elevator.

Terrorists have proven that we can't protect 1368 ft of skyscraper. What makes you think we can protect the 40,000 ft of cable within airliner cruising altitude? The best (best? how about worst?) they can do is pull the cable out of geosynch orbit. There isn't much you can do to create more publicity than destroying a 40 billion dollar space elevator. OK, there is one way: drag it behind a plane and wrap it around the earth twice. I think that pegs the publicity meter.

Real life is not a movie, you cannot drag a satellite down with a plane even with a cable attached. The plane can only hit on the cable, and the cable may break or not, either way the plane will probably be cut open by the cable and crash.You cannot use a plane to hold and drag such a long cable anywhere and stay flying. If the cable is cut, the earth-side cable will fall, the sky-side cable and the rock will slowly fly away from Earth. And if there is enough rocket fuel on the rock, it can be stabilized quickly and stay in orbit. It won't be destroyed so simply. By adjusting the configuration of the "counter-weight", you can balance the orbiting part no matter how much of the rope remains dangling from the rock. Any kind of failsafe mechanism that breaks the rope when there is too much tension will keep the space-part of the elevator safe from this kind of attack.

To drag it down, you have to take over the ground installation and then reel the rope in, assuming you have the machinery to reel in such a cable, and the operators on the other side do nothing, and the rest of the world does nothing. One cruise missile or jet fighter can break the cable relatively close to the ground and the operator in space can compensate to maintain orbit. In the worst case, you can always cut the cable on the upper end, retract the counter-weight, which leaves you with a rock on geosynch orbit and some re-wiring to do later.

In fact you could do nothing. Most of the cable's strength goes into holding up the cable. The cable is therefore a lot thinner and weaker near the ground then higher up. If you tried to reel it in, *snap* before the orbit is significantly changed. The same thing happens with any plane scenario that doesn't snap the cable outright.

I think the beauty of the design is that by keeping the cable in tension, the space elevator is fail-safe with respect to damage to the ground and cable.

A plane doesn't hit the cable, a hook does. The hook is mounted on a winch, free spinning at first, then gradually slowed down as the cable accelerates. Once the winch stops the plane is dragging the 62,000 mile cable behind it toward the west, decelerating the space elevator. Authorities shoot down the plane, but now the satellite is rotating slower than the earth. It's just a matter of time until the earth wraps the cable around itself.

Clever. I hadn't thought of that.

That's assuming nobody is looking, and no stabilizing mechanism is present anywhere in the system. Once the offending plane is down, just a couple rocket boost and the satellite is back to normal. You bet there will be enough motion detection (GPS, laser, gyroscopes, etc) that any unusual stress to the system will be detected right away.

The plane attack given above doesn't work, and shows a serious lack of understanding of physics, both "orbital" and conventional. A cable or rope can exert force only in the direction of the cable itself. Given that the said cable is thousands upon thousands of miles long, to exert any force at all in the direction of the orbit itself (necessary to lower or raise the energy of the orbit) is going to require that the cable be pulled a significant distance so that the sin(x) of the resulting angle has any significant component... and even so, the vast majority of force a plane could exert would still be "down" from the point of view of the space station. The net result is that if it were strong enough, the plane might pull the space station into a slightly lower orbit, but as soon as the plane releases, it will return upwards, being put into an elliptical orbit with the more-or-less the same energy.

But, and this is a big "but", no conceivable plane could exert enough force to make any significant difference. The plane will never make it to the ground... instead, if it insists on hooking itself to the cable, it will find itself suspended in the air and quite possibly pulled out into space itself!... assuming the plane would even hold together, which it won't because it might as well be made of tissue paper compared to the diamond the space elevator is made out of. To tow something, you need a solid connection from the engine to the tow point (try towing a car by attaching to the bumper, but not a car you care about...), and you'd basically have to custom-build a plane for this application, which isn't technically impossible but would be a serious undertaking for even the US military; I don't think terrorists will be able to scrape this together.

It would almost be funny if somebody tried this. The elevator would be fine but I doubt the perpetrator would be.

These are big forces involving orbital mechanics. Your intuition is nearly useless. This isn't your daddy's elevator. There's damn near nothing you can do near the ground to take out the space elevator permanently. You need to be really high up to do any real damage. Now, if only we had an elevator that could take us that high... point is that of course it can still be destroyed as other contributors note in the following paragraphs. But ground security will go a long ways towards preventing that, and we can realistically have extremely paranoid security while we use the first one to build two more.

The obvious way for a terrorist to sabotage this system big time is to use either a bomb in a cable car or a redirected space rock to cut the cable close to the orbital station. At that point what you have is many tons of cable being reeled in by tidal forces. Once the upper end is decelerated sufficiently, gravity kicks in to add a few ergs of its own. Result is a very loud bang.... and little more; all proposed space elevators that I've seen burn up in the atmosphere and become little more then hot whiffs of carbon dioxide long before they hit the ground.

But nowhere near so bad as if you simply steer a big rock onto an earth collision course. Space police would have their hands full ... if it weren't for the fact that we'll likely destroy ourselves with biotech long before there are any SpaceElevators ...

One of the proposals I have seen for a SpaceElevator (I think the NIAC one but I'm not positive) called for the material of the elevator to be made from a material which will fragment and "burn up" when exposed to a certain amount of heat. The idea was that if the cable was cut, in any way, near the orbital station it would burn up producing a fairly spectacular light show but, no real damage. -- Tim

Don't forget the problem with meteors and space junk in general. NASA's experiment a few years ago with a tether 4 km long from the shuttle ended when it was unexpectedly severed by something.

So, really we need to worry about 'space pollution'... the fact being that no-one seems to care an awful lot about anything that is likely to stay in orbit. Why would we, when the international space station is the largest thing in orbit? I guess the question is, have we shot ourselves in the foot with the results of space programs for the last 50 odd years?

One advantage of a SpaceElevator is that you can use space-generated electricity to power the climb. Another is that you can pause halfway up. Try that with a Saturn V!

Why not use a rocket to power the elevators in any skyscraper? The efficiency gains are the same.

No, you misunderstand - the SpaceElevator is not more efficient - it takes the same amount of energy to lift the mass up to orbit, but the SpaceElevator can a) use easily-generated electricity and b) use a traction system to crawl up the line, and so be able to pause indefinitely at a given altitude.

No, I don't misunderstand; yes, it takes the same amount of energy with an ideal propulsion system. Your statement of 'easily-generated' is an admission of this. It isn't a question of how much energy is required to get to height 'x', but a question of how much energy is wasted over and above that. In other words, I should have been clearer and stated that you don't get rocket fuel for zero energy cost. :)

That you don't need to lift the propellant is where the energy saving comes from. Almost the entire takeoff mass (95%?) of a rocket is fuel, so the vast majority of the fuel is used just for lifting fuel, and what holds that fuel. That's why multistage rockets are used. There is no such wastage on a space elevator.

But you do need to lift the propellant. It doesn't have to be fuel, but somehow you've got to get power to the lift. Unless you've got a superconductor in there you'll see inefficiencies of power transmission.

How about a superconductor? We're talking a pretty significant nanoengineered structure already. :-)

Of course. The easiest defense is to make the target not worth attacking. For example, terrorists are unlikely to blow up my dog-house. The value of the space elevator as a target exceeds the cost of effectively defending it.

The U.S. Navy has 12 aircraft carriers. Each of them cost about 5 billion dollars to build; their total cost is more than it would cost to build 2 SpaceElevators. Why would the SpaceElevator be any more difficult/costly to defend than these 12 aircraft carriers?